As gas sensors for fuel efficiency improvement and combustion control of internal combustion engines (including automotive engines), there are known oxygen sensors and air-fuel ratio sensors, each adapted to measure the concentration of oxygen in a gas under measurement (such as intake gas or exhaust gas). Among others, commonly used is a gas sensor having a plate-shaped gas sensor element that extends in the direction of an axis and includes, on a front end side thereof, a detection portion for detecting a specific gas component in a gas under measurement. In this type of gas sensor, a porous protective layer is formed around the detection portion so as to protect the detection portion from thermal shock by contact with water in the gas under measurement.
It has been common to form the porous protective layer by preparing a slurry containing a raw material of the porous protective layer, and then, spraying the slurry onto the detection portion as disclosed in JP 2007-33374A (FIG. 5) or dipping the detection portion into the slurry as disclosed in JP H08-50114A (FIG. 4). However, such spraying or dipping leads to variation in the thickness of the porous protective layer. There is thus developed a technique to form the porous protective film with a controlled thickness by injecting the slurry into a forming mold in which the detection portion is placed as disclosed in JP 2013-217733A (FIG. 2).